This invention relates to the field of broadband amplifiers, especially those amplifiers used in cable television and telecommunications systems. The amplifier circuit of the present invention has an extremely uniform response over a very wide range of frequencies.
A cable television system requires an amplifier capable of providing a desired amount of amplification over a wide bandwidth, typically from 30 MHz to 1000 MHz. It is desired that the amplifier have a gain which remains within +/-1 dB (and preferably +/-0.5 dB) of a desired value over the frequency range of interest. Of course, the gain of conventional amplifiers varies with frequency. A conventional amplifier, operated alone, simply cannot achieve the desired flatness over the five-octave frequency range specified above. In order to achieve the desired flatness over the desired frequency range, it has been necessary to use multiple compensation circuits and/or additional amplifiers.
One type of amplifier which is especially suitable for use in cable television systems is the monolithic microwave integrated circuit (MMIC). Such devices are commercially available; a typical MMIC is the MSA-1104, sold by Hewlett-Packard. The latter MMIC is designed for use over a broad frequency band. Nevertheless, when used from 30 MHz to 1000 MHz, the latter MMIC has a gain which varies by about 2.1 dB over this range. FIG. 4a provides a graph of gain versus frequency for this MMIC. The graph is negatively sloped, indicating that the gain of the amplifier decreases towards the upper end of the frequency range. In order to use the MMIC throughout the desired frequency range, it is necessary somehow to compensate for this variation so that the frequency response is essentially flat.
It has been known to compensate for a sloped frequency response curve by adding a circuit having a transfer function that is negative with respect to that of the original amplifier. Such a compensation circuit could comprise "lumped" elements which include capacitors, inductors, and resistors. An example of such a compensation circuit is shown within the dotted lines in FIG. 2. But it turns out that this slope compensation circuit alone does not yield the desired flatness of frequency response because it presents a non-constant impedance across the desired bandwidth.
The present invention provides a circuit which achieves the desired flatness of response over the five-octave frequency range mentioned above. The invention combines either a conventional MMIC, an MIC (i.e. a microwave integrated circuit, in which the active device is monolithic but is mounted on a substrate having other discrete components), or a discrete transistor with a slope compensation circuit, and with a network of microstrips, so as to provide the necessary frequency compensation.